Conventional chemotherapy has been thought to act through the direct killing of tumor cells. However, accumulating evidence indicates that immune competence is crucially required for chemotherapy efficacy. It is expected that loss of this immunity during chemotherapy has a negative effect on its efficacy. Therefore, the impact of chemotherapy on anti-tumor immunity needs further investigation in order to rationally design combinatorial regimens for conventional chemotherapy. In the preliminary studies, we have demonstrated that repeated chemodrug gemcitabine (GEM) treatment promoted the expansion and differentiation of immunosuppressive Ly6Chigh monocytic-MDSC (M-MDSC). Tumor-derived soluble factors, such as GM-CSF and soluble ICAM-1(sICAM-1), were up-regulated upon chemo-drug treatment. In tumor-free nave mice, GEM treatment induced the immunosuppressive activity of Ly6Chigh myeloid cells in the bone marrow (BM). The chemokine CX3CL1 and its receptor CX3CR1 expression levels were elevated in the BM. Higher levels of mitochondrial reactive oxygen species (mtROS) were also observed in tumor cells and BM stromal cells following chemotherapy. Furthermore, chemotherapy induced NF-?B activation leading to the hyperproduction of GM-CSF by tumor cells. Based on these preliminary data, we hypothesize that chemotherapy enhances the production of mtROS in tumor cells and BM stromal cells, which increases the expressions of GM-CSF and sICAM-1 as well as CX3CL1 leading to the enhanced immunosuppression of M-MDSC in the TME. These hypotheses will be addressed by two Specific Aims. Aim 1 defines the roles of GM-CSF and sICAM-1 in chemotherapy-induced differentiation of immunosuppressive Ly6Chigh myeloid cells and the mechanisms underlying mtROS and upregulation of GM-CSF and sICAM-1 in tumor cells; Aim 2 determines the mechanisms by which host cell- derived chemokine CX3CL1 regulates the accumulation and immunosuppressive function of inflammatory Ly6Chigh myeloid cells in the BM. The findings from these studies will allow us to gain a better understanding the underlying mechanisms by which ongoing inflammation following multi-dose clinical regimens of chemotherapy modulates anti-tumor immunity, and rationally design a novel therapeutic approach by combining chemotherapy with mitochondria-targeted antioxidants for cancer treatment.